The efficiency of an optical fiber as a signal transmission conduit is considerably affected by the precision in transferring the light between the optical fiber and an optoelectronic converter such as a photodetector or laser that converts the light signals to electrical signals or electrical signals to light signals respectively. There is generally a considerable mismatch between the light radiation or reception patterns of optical fibers and the light radiation or reception patterns of optoelectronic converters. The alignment tolerances are very small thereby requiring housings that can maintain precision alignment in service.
The simplest optical subassembly arrangement is to butt the face of the optical fiber to the optoelectronic converter. In this type of arrangement the light transfer efficiency is so low as to seriously restrict the uses.
The use of small lenses of various types has been employed in the art to help in accommodating the light radiation or reception mismatch.
An illustration of the state of the art of the use of small lenses in optoelectronic conversion subassemblies is provided in "Microlenses to improve LED-to-fiber optical coupling and alignment tolerance" by J. G. Ackenhausen in Applied Optics Vol.18 No. 21 November 1979.
As the art is progressing, many optoelectronic conversion subassemblies are becoming arrangements wherein a housing supplies rigid alignment among the optical fiber, the lens and the optoelectronic converter. An illustration of this type of optical subassembly is described in "Low-Loss Laser Diode Module Using a Molded Aspheric Glass Lens" by Kato et al in IEEE Photonics Technology Letters Vol.2 No. 7 Jul. 1990 pages 473-474.
Optical subassembly constructions of this type however have the limitations of requiring carefully selected materials for the housings and the lenses for both optical performance and mechanical stability in addition to being able to withstand high temperatures while the parts are permanently attached or fixed in position during the alignment steps of the assembly process. High precision machining of the housings and lenses to establish axially aligned locations is required. Once constructed, intricate alignment operations in three dimensions are necessary to achieve good power transfer.
The use of molded plastic in optical subassemblies has been an attractive goal in order to reduce machining cost. One illustration is in IBM Technical Disclosure Bulletin, Vol.34, No. 7B, Dec. 1991, Page 230 wherein there is described a tapered bore in a plastic housing that is designed to hold an optical fiber end in fixed relationship to an hermetically sealed optoelectronic converter that has a lensed can.
In U.S. Pat. No. 4,616,899 an optoelectronic subassembly is arranged with a moldable plastic housing wherein portions of the housing material serve as lenses. The structure requires a complex alignment procedure. An illustration of the complexity of alignment along all three X, Y and Z axes as practiced in the art is shown in U.S. Pat. No. 5, 029,965.
There is a need in the art for the ability to produce optical subassemblies with reduced machining, greater flexibility in material selection and simpler alignment to achieve an inexpensive but nonetheless functional and durable assembly.